Silicon Valley startup Pinterest raised $100 million from a group led by Japanese online retailer Rakuten, valuing the company at about $1.5 billion and underscoring the huge investor appetite for social-networking companies.

Palo Alto-based Pinterest, an online scrapbook where users can "pin" images and follow others, has grown from less than 1 million users in May 2011 to about 20 million in April, according to IT research house comScore.

The 3-year-old company, led by co-founder Ben Silbermann, had been seeking financing for several weeks, but many investors were wary of the billion-dollar valuation, venture capitalists said.

That valuation puts Pinterest beyond the reach of all but a handful of potential acquirers, probably putting it onto a path for an eventual IPO rather than an acquisition.

Rakuten, which offers services ranging from online golf reservations to Internet banking, was joined in the investment by existing Pinterest shareholders Andreessen Horowitz, Bessemer Venture Partners and FirstMark Capital.